The problem conventional techniques and the invention try to solve is explained in the order of the following (1) single board optical disc and (2) two-disc bonded optical disc.
(1) Single Board Optical Disc
A so-called compact disc (CD) is well known as an optical disc that records a music signal by performing 44.1 kHz sampling and 16-bit quantization for a music signal audible band.
On the contrary, for an application in studios and so on, a music signal is recorded as a sampling frequency of 88.2 to 96 kHz and the number of quantized bits of 20 to 24 for up to an ultrasound band (approximately 20 to 50 kHz band) exceeding the music signal audible band. However, an expensive application apparatus is required for this recording and reproduction.
The art rejecting unimportant audible parts of sampling and quantized data for a music signal audible band, compressing irreversibly an original data with one kind of compression algorithm, and recording and reproducing on a recording medium the constant amount of information per apparatus time of music play time, is known.
The art compressing irreversibly an original data by using a video space correlation, using time correlation, and rejecting unnecessary visual data in relation to data of video signal sampling and amplitude quantization, is also known.
To compress irreversibly data like this, there are methods for making the amount of video information per apparatus time constant, and for changing the amount of video information per apparatus time depending on the video condition; both arts are known.
In addition, the art recording and reproducing irreversibly compressed data on an optical disc using the former method that makes the amount of video information per apparatus time constant, is known. Further, the art recording and reproducing on an optical disc irreversibly compressed data wherein the amount of information per apparatus time of video play time is changed depending on a video condition using the latter method for changing the amount of video information per apparatus time depending on a video condition, especially the art reproducing with buffer memory, pickup kick wait (regenerating wait operation), and search function, are known. To store a large amount of data efficiently, a hard disc drive (HDD) for use with a computer may record the data by compressing reversibly, with a naming such as so-called lossless compression and so on, and at reading, expand complementarily the data to be regenerated; this art is known.
Furthermore, the main art of a high-density optical disc recording video and/or music information at a density 3 to 8 times higher than that of a CD for general consumers, is known.
The art wherein a first area that is the inner side of an optical disc is a CD voice signal recording area, and a second area that is the outer side of the first area is a video signal recording area for recording at a high density a analog FM modulated video signal, is called CD-V (CD-VIDEO) and known.
Still, the disc wherein the first area that is the inner side of a high-density recording disc has CD recording density (low-recording density), and the second area of its outer side has high-recording density, is, for example, described in Japanese Patent Laid-Open No. 168449/1994 and known. A system is designed such that, against a low-density disc reading laser wavelength (780 nm ), a high-density disc is read with a short wavelength laser (635 nm). At this time, the pit depth of a disc is determined in relation to 0.25 times reading laser wavelength. In other words, it is suitable to make the pit depth of high-density disc shorter than that of a low-density disc.
Also, even if the pit depth is not defined as a standard, it is common that a reproducing signal characteristic with a standardized normal optical pickup is defined with a range; and the pit depth is defined equivalently with a range. Further, in general, the mechanical accuracy is reduced for a low-density disc, and is strictly defined for a high-density disc.
In relation to music signal recording and reproduction to an optical disc, improvements in a voice signal frequency band (i.e. in a sampling frequency) and in its amplitude axis accuracy (i.e. in the number of quantized bits), are required.
High-density digital data that is a music signal having a sampling frequency of 88 to 96 kHz, and the number of quantized bits of 20 to 24 for up to an ultrasound band (approximately 20 to 50 kHz band) exceeding a music signal audible band, has the amount of data per apparatus time 2.5 to 3.3 times larger than that of a basic digital data locating 44.1 kHz sampling and 16-bit quantization.
When this high-density digital data is recorded and reproduced on an optical disc like a CD, it is necessary to offer 2.5- to 3.3-times improvement in storage capacity per optical disc, in order to equalize play time per optical disc with that of a CD. The contents of the main art of a reproducing apparatus reproducing a high-density optical disc on which this high-density digital data is recorded, is known as described above.
However, such high-density optical disc cannot be reproduced with a conventionally available reproducing apparatus (e.g. CD player). For this reason, music publishers must publish two kinds of the same music program source for a CD and a high-density optical disc, i.e. perform double inventory.
Also, means for concretely solving this double inventory to make single inventory, has not been disclosed.
(2) Two-disc Bonded Optical Disc
A multilayer optical disc having a plurality of recording layers by bonding together two optically transparent substrates on which a convex and recessed shape information pit and reflective layer are formed, in order to increase the recording space of an optical disc so as to read optically information formed on a helical (spiral or concentric) information track, is disclosed in Japanese Patent Laid-Open No. 223030/1990 and Japanese Patent No. 27815/1986. One example of a multilayer optical disc disclosed in these is shown in FIG. 13.
FIG. 13 is a cross-sectional view showing one example of a conventional multilayer optical disc, and is a diagram taken along a cutting surface through the center of an optical disc. The horizontal direction in the same diagram shows a radius direction of the optical disc; and the vertical direction shows a thickness direction of the optical disc.
In an optical disc 50 of FIG. 13, a first recording layer 52 on which information is recorded by a convex and recessed shape information pit is provided on a disc substrate (optically transparent first substrate) 51; and a first reflective layer 53 is provided on the first recording layer 52. Likewise, a second recording layer 56 on which information is recorded by a convex and recessed shape information pit is provided on a disc substrate (optically transparent second substrate) 55; and a second reflective layer 57 is provided on the second recording layer 56.
Each recording layer side of the first substrate 51 and the second substrate 55, is bonded through a bonding layer 54.
The optical disc 50 has a thin disc substrate for high-density recording, reducing various optical aberrations due to disc substrate thickness.
For example, in order that storage space per recording layer is 4 times the 1.2 mm disc substrate thickness of a conventional CD, the first substrate 51 and the second substrate 55 have approximately 0.6 mm thickness, respectively, and then the two disc substrates are bonded together to have recording space 8 times that of a CD.
As shown in FIG. 13, the first recording layer 52 is formed on the first substrate 51 and is provided in proximity to the bonding layer 54, while the second recording layer 56 is formed on the second substrate 55 and is provided near the bonding layer 54. In other words, two recording layers are provided in proximity to the thickness direction center of the optical disc 50.
The reflective rate of the first reflective layer 53 is a low reflective rate (e.g. 30%), while that of the second reflective layer 57 is a high reflective rate (e.g. 95%). A reproducing laser light is irradiated from the first substrate 51 side as shown in FIG. 13, and is gathered on the first recording layer 52 or the second recording 56 to read information of each recording layer.
Such reproducing apparatus reproducing a high-density optical disc can naturally reproduce the information on the first recording layer 52 on the second recording layer 56, recorded at a high density; and can generally regenerate a low recording density optical disc like a CD.
To reproduce both of a high-density and low-density optical discs, the reading optical head in a high-density optical disc reproducing apparatus has separately the optical system of a high-density disc optical disc and that of a low-density optical disc, or adopts a two-focus hologram-applied optical system; therefore, according to each optical disc, the size of a laser light spot is optimized, and the difference in the disc substrate thickness is corrected.
However, there is a problem that a high-density optical disc cannot be reproduced by a conventional low-density optical disc reproducing apparatus.
In a low-density optical disc reproducing apparatus, this is caused by a too large diameter of a laser light spot gathered on the recording layer, and the great difference in thickness between a high-density and low-density disc substrates.
Ideally, it is desirable that a low-density area A and a high-density area B coexisting on one information recording disc, meet standard A based on that the entire disc surface is low-density area A, and standard B based on that the entire disc surface is high-density area B, respectively.
In a disc producing method for recording a phase pit for mass replicate, the thickness of the photosensitive agent on an unexposed original disc prepared before recording, determines a pit depth. It is generally difficult to change the thickness of the photosensitive agent on one original disc in a 5% or more step-by-step manner. In a normal method, the thickness of the photosensitive agent on one original disc is uniform. When the low-density area A and the high-density B coexist on one disc, there is a problem of which one the pit depth is produced for. The example that the digital signals of the low-density area A and the high-density area B coexist on one disc, is described in Japanese Patent Laid-Open No. 168449/1994 mentioned above, but a means for solving the above problems and this problem has not been covered at all.
For the problem of when the low-density area A and the high-density B coexist on one disc, which one the pit depth (or groove depth) is matched to, the invention combines a method for defining a pit depth in relation to signal output, with a method for defining mechanical and optical accuracies in relation to the disc mechanical accuracy. In the standard A based on that the entire disc surface is low-density A and the standard B based on that the entire disc surface is high-density B, the standard B will generally be determined chronologically in later years. The standard B generally uses a short wavelength laser, having a pit depth shallower than that of the standard A by an approximately short wavelength.
The standard B defined chronologically in later years is for high-density recording in consideration of use of a relatively short wavelength laser and a relatively high aperture lens; and needs more improvement in disc mechanical and optical accuracies than those of the standard A. Considering these requirements and press technology improvement, the disc mechanical and optical accuracies of the standard B are determined to be higher than those of the standard A.
From the above reasons, a low-density and a high-density optical discs on which the same contents of program (information) is recorded, are produced and sold separately, resulting in various inconvenience between manufacturers and users.
This invention is made in consideration of the above problem, and aims to offer an information recording disc having a low-density recording area and high-density recording area with extremely much recording space.